1. Field of the Invention
The present invention relates to a regenerative refrigerator that uses a refrigerant gas such as helium gas and has a regenerator containing a regenerator material.
2. Description of the Related Art
A regenerative refrigerator that uses a refrigerant gas such as helium gas and has a regenerator containing a regenerator material is used to attain a cryogenic temperature of approximately 4 K, for example. Further, refrigerators such as Gifford-McMahon (GM) refrigerators and pulse tube refrigerators are used as regenerative refrigerators.
The regenerative refrigerator performs refrigeration by causing a refrigerant gas to expand adiabatically and storing the cold (cold heat) generated at that time in a regenerator material. Therefore, the regenerative refrigerator includes a cylinder and a regenerator tube connected to the cylinder, and has a regenerator material for storing cold provided inside the regenerator tube.
The regenerator material needs to have high specific heat at cryogenic temperatures that are operating temperatures. In general, however, the specific heat of metal such as lead decreases sharply with a decrease in temperature at cryogenic temperatures lower than or equal to 15 K. Therefore, a magnetic regenerator material having higher specific heat than lead at temperatures lower than or equal to 15 K, such as HoCu2, is used as a regenerator material. However, the magnetic regenerator material, which has high magnetic susceptibility at 15 K or below, may generate magnetic noise. This makes it necessary to provide a magnetic shield around the regenerator tube, thus increasing the manufacturing cost.
On the other hand, helium, which is used as a refrigerant gas, has high specific heat at cryogenic temperatures that are operating temperatures. Therefore, a gas regenerator material formed of helium gas may be used as a regenerator material for cryogenic temperatures lower than or equal to 15 K. (See, for example, Japanese National Publication of International Patent Application No. 2006-524307.) Using a gas regenerator material formed of helium gas eliminates the necessity of providing a magnetic shield around the regenerator tube, and thus makes it possible to reduce manufacturing cost.